Liquid crystal display devices possess several advantages. Liquid crystal display devices can be light weight and have a relatively small thickness. Such devices can also be inexpensive, exhibit low-power consumption driving and can be compatible with integrated circuits. Because of these and other advantageous properties, liquid crystal display devices are currently used in notebooks, personal digital assistants (PDAs), cell phones and color televisions.
A liquid crystal display device typically includes a color filter array as a lower substrate and a TFT array as an upper substrate. The lower substrate can include a black matrix, a color filter and an ITO pixel electrode, and the upper substrate can include an active circuit portion composed of a liquid crystal layer, a thin film transistor and a capacitor layer and an ITO pixel electrode.
The color filter can be produced by forming red, green and blue pixels and a black matrix on a glass substrate, wherein the pixels and the black matrix are formed using photosensitive resin compositions comprising fine pigment particles dispersed therein. The black matrix plays a role in blocking uncontrolled light that is transmitted through the elements other than the transparent pixel electrodes of the respective substrates to prevent a reduction in contrast caused by light passing through the thin film transistor. The primary role of the red, green and blue pigment layers is to transmit light of a particular wavelength from white light, thereby creating these respective colors.
Generally, color filters are produced by dyeing, printing, pigment dispersion, electrodeposition, and other processes.
In a dyeing process, a color filter is produced by forming a black matrix on a glass substrate, applying a photosensitive solution, which is prepared by sensitizing a natural photosensitive resin (e.g., gelatin) or a synthetic photosensitive resin (e.g., an amine-modified polyvinyl alcohol or amine-modified acrylic acid) with dichromic acid, exposing the photosensitive solution to light through a photomask, followed by developing or dyeing with a masking coat and an acidic dye. The dyeing process, however, uses a number of colors on one substrate, which requires a resist dyeing process whenever colors are changed. This resist dyeing process can complicate and lengthen the time required for the overall procedure. Further, although dyes and resins commonly used for the dyeing process have good dispersibility and high visibility, they can have poor resistance to light, moisture and heat. Heat resistance of dyes and resins is an important property of color filters.
Korean Patent Publication Nos. 91-4717 and 94-7778 are directed to the use of an azo compound and an azide compound as dyes, respectively. However, these dyes have the disadvantages of poor heat resistance and low durability as compared to pigments.
In a printing process, a color filter is produced by printing using an ink, which is prepared by dispersing a pigment in a thermosetting or photocurable resin, and curing the printed ink with heat or light. Accordingly, printing processes can be economically advantageous in terms of material costs when compared to other processes. Printing processes, however, can also require the accurate alignment of three-color filter patterns, which can make it difficult to form precise and fine images and can result in the formation of a non-uniform thin film layer.
Examples of methods for producing color filters using an ink-jet technique are set forth in Korean Patent Laid-open Nos. 95-703746 and 96-11513. However, these methods use dye-type color resist compositions which are sprayed through a nozzle to print fine and accurate colors. Thus these methods have the disadvantages of low durability and poor heat resistance as in dyeing processes.
Korean Patent Laid-open Nos. 93-700858 and 96-29904 are directed to methods for producing color filters using electrodeposition. Electrodeposition can be employed to form a precise pigmented network. Electrodeposition processes can be advantageous because a pigment is used, which can exhibit desirable heat and light resistance. However, as the size of pixels decreases and electrode patterns becomes finer, electrodeposition processes can form colored speckles as a result of an electrical resistance at both ends of the pattern, or can increase the thickness of colored films, thus limiting the use of electrodeposition to the production of color filters requiring a high degree of precision.
In a pigment dispersion process, a color filter is produced by coating a photopolymerizable composition comprising a pigment on a transparent substrate, exposing the coated substrate to light to obtain a desired pattern, stripping the unexposed portion using a solvent, thermally curing the exposed portion, and repeating the process. Advantages of pigment dispersion processes include improved heat resistance, an important property of color filters, high durability and uniform film thickness.
Because of these advantages, pigment dispersion processes are widely used for the production of black matrices. For example, Korean Patent Laid-open Nos. 92-702502 and 95-700359 and Korean Patent Publication Nos. 94-5617 and 95-11163 are directed to processes for producing color resists using pigment dispersion.
A black matrix produced by a pigment dispersion process can be essentially composed of the following components: i) a polymeric compound (i.e. a binder resin) that acts as a support and maintains a constant thickness, and ii) a photopolymerizable monomer that responds to light upon exposure to form a photoresist phase. A black matrix can also be produced using a photosensitive resin composition comprising a polymeric compound, a photopolymerizable monomer, a pigment dispersion, a polymerization initiator, an epoxy resin, a solvent, and other additives. Various binder resins can be used for pigment dispersion processes, including a polyimide resin (Japanese Unexamined Patent Publication No. Sho 60-237403), photosensitive resins composed of an acrylic polymer (Japanese Unexamined Patent Publication Nos. Hei 1-200353, Hei 4-7373 and Hei 4-91173), a radical polymerization type photosensitive resin composed of an acrylate monomer, an organic polymeric binder and a photopolymerization initiator (Japanese Unexamined Patent Publication No. Hei 1-152449), and photosensitive resins composed of a phenolic resin, a crosslinking agent having an N-methylol structure and a photoacid generator (Japanese Unexamined Patent Publication No. Hei 4-163552 and Korean Patent Publication No. 92-5780).
However, although the use of a photosensitive polyimide resin or a phenolic resin as a binder resin in a pigment dispersion process can be advantageous in terms of high heat resistance, such binders can suffer from the drawbacks of low sensitivity and development with an organic solvent. Further, conventional systems using an azide compound as a photosensitizer can offer the problems of low sensitivity, poor heat resistance and attack by oxygen upon exposure.
To solve these problems, formation of an oxygen barrier film and exposure to an inert gas can be considered. However, these additional steps require complicated processing and incur increased equipment expense. Although a photosensitive resin forming images using an acid generated upon exposure is highly sensitive and is not attacked by oxygen upon exposure, heating is necessitated in the course of exposure and development steps and formation of a pattern is highly dependant on heating time, which makes the management of the processing difficult.